DAQ background noise

[John_Amery]

Hi folks,

I'm using Daq USB-6211. Platform is MAC OS X. Labview version: 8.5. 

The Daq sample at 250 HZ and each time take 250,000 data. So basically it sample 1 sec and continue sampling without stop. It's in differential mode. 

The measurement starts with measuring the background noise of an ordinary coil. The coil is made up of thin copper wire with resistance 40 khm and inductance 0.1 Henry. There is a SR-560 preamplifier in between the Daq and the coil. Gain is 1000. it's in low noise mode and no filter is applied.

Now the energy spectrum shows there are 10K, 20K, 30K, 40K and 50K noise in it. They are very regular and I suspect they are produced by the Daq itself. It seems like its intrinsic clock? I really need to get rid of these noise to proceed.

Thanks,

John

[John_Amery]

The sample rate is 250 kHz...

[hecmar.arreola]

It is kind of a shot in the dark but I usually try and the chassis of each device wired together to a common ground.  That tends to clean up a lot of my noise...even if your signal wire termination IS in differential mode.

Also, if your cable has shielding and it is possible try to have the shielding only grounded on 1 end.  Sometimes having shielding connected on both ends of a cable results in noise because there is a potential difference between the two.  If one side of the shielding is disconnected then there won't be any current.

[John_Amery]

Thanks. Your reply reminds me of the ground loop issue. But I can't totally agree that connected every device ground together can help that much. To my knowledge, the ground loop issue may still show up as different device has different ground paths. But I'll give a try anyway. 

In fact, both preamp and the Daq are in differential mode which means the two leads of the coil are floating. They are both BNC cables. 

John

[johnsold]

John,

First, some questions about things in your original post which did not make sense to me.

1. You said you are sampling at 250 Hz and reading 250000 samples in one second.  So I presume you meant that you are sampling at 250 kHz.

2. Is the resistance of your coil 40 ohms or 40000 ohms?

How do you prevent any magnetic coupling to the coil during your background measurements?

Can you post the spectral data or at least an image? What does the signal look like in the time domain?

It should be easy to isolate any internal noise from the DAQ. Terminate the inputs with a metal film resistor with a value equal to the coil resistance. Shield the resistors if the environment is noisy. Measure the spectrum.

Lynn